One of our farriers said that covering sunburned noses in Swat helps the skin heal quickly, but I haven't had the chance to try it.
To prevent my mare from getting sunburned, I use a cheap brand of 50 SPF baby sunscreen. We both use it. ;) It's best to get the kind that is sweat and water resistant. I've been putting it on her pretty much every day this summer, but every other day usually works fine.
There's also a type of sunscreen that's made specially for horses, but I forget its name... :P
Like other people have said, there is sunscreen you can buy, you can use normal or special ones made for horses. You won't believe it but there is now fluro horse sun screen that you can put on your horse's nose! Like yellow, green and pink!
Alas, my horsie has the same problem! I constantly find scabbies on her nose. For a little while I was using Ihle's Paste (zinc), and now I'm just slathering her nose in regular 30 SPF human sunscreen.